Exchange Server TLS guidance Part 2: Enabling TLS 1.2 and Identifying Clients Not Using It

Overview

In part 2 of our Exchange Server TLS Guidance series we focus on enabling and confirming TLS 1.2 can be used by your Exchange Servers for incoming and outgoing connections, as well as identifying any incoming connection which is not utilizing TLS 1.2. The ability to identify these incoming connections will vary by Windows Server OS version and other factors. Part 2 will not cover disabling TLS 1.0 or TLS 1.1, nor disabling older cipher suites from being used. Part 3 of the TLS guidance series will go into detail on those topics.

Assumption

For Part 2 of our TLS guidance series we assume you have already audited your on-premises Exchange Servers and applied all updates called out in Part 1: Getting Ready for TLS 1.2. Please perform the activities called out in part 1 if you have not prior to moving forward with any configurations outlined in part 2.

Enabling TLS 1.2

The method used to enable TLS 1.2 varies by the version of the Windows Server operating system. Some versions of Windows Server have TLS 1.2 enabled by default while others do not. Our steps will, regardless of the OS’ default state, configure TLS 1.2 so it is enabled and available for incoming (Server) connections and outgoing (Client) connections.

From part 1 you should be familiar with the various components Exchange Server relies on such as Schannel, WinHTTP and .NET. Unless stated otherwise the same registry paths are used across all supported Windows Server operating systems.

Enable TLS 1.2 for Schannel

All Windows Server versions

TLS protocols are enabled or disabled in Windows Schannel by editing the Windows Registry. Each protocol version can be enabled or disabled independently. You don't need to enable or disable one protocol version to enable or disable another protocol version.

The Enabled DWORD registry value defines whether the protocol version can be used. If the value is set to 0, the protocol version cannot be used, even if it is enabled by default or if the application explicitly requests that protocol version. If the value is set to 1, the protocol version can be used if enabled by default or if the application explicitly requests that protocol version. If the value is not defined, the operating system’s default value will be used. We recommend configuring the value to have a consistent state across your servers.

The DisabledByDefault DWORD registry value defines whether the protocol version is used by default. This setting only applies when the application doesn't explicitly request the protocol versions to be used. If the value is set to 0, the protocol version will be available for use by default. If the value is set to 1, the protocol version will not be available for use by default. If the value is not defined, the operating system’s default value will be used. We recommend configuring the value to have a consistent state across your servers.

For example; consider what would happen if TLS 1.2’s values were set to a combination of Enabled and DisabledByDefault both set to a value of 1. In this example an application could only use TLS 1.2 if the application specifically called for TLS 1.2. If the application did not specifically call for TLS 1.2, then it would not be able to use TLS 1.2 as even though the protocol is enabled, it is not in the default list of available protocols.

To enable TLS 1.2 for both server (inbound) and client (outbound) connections on an Exchange Server please perform the following.

Enable TLS 1.2 for .NET 3.5

This step is only required for Exchange Server 2010 installations where .NET 3.5 is relied upon. Exchange Server 2013 or later installations may skip this step unless you have additional applications on the server utilizing .NET 3.5 which must be able to use TLS 1.2.

The SystemDefaultTlsVersions registry value defines which security protocol version defaults will be used by .NET Framework 3.5. If the value is set to 0, then .NET Framework 3.5 will default to using SSL 3.0 or TLS 1.0. If the value is set to 1, then .NET Framework 3.5 will inherit its defaults from the Windows Schannel DisabledByDefault registry values. If the value is undefined, it will behave as if the value is set to 0. By configuring .NET Framework 3.5 to inherit its values from Schannel we gain the ability to use TLS 1.2.

From Notepad.exe, create a text file named NET35-UseSchannelDefaults.reg.

Enable TLS 1.2 for .NET 4.x

This step is only required for Exchange Server 2013 or later installations where .NET 4.x is relied upon.

The SystemDefaultTlsVersions registry value defines which security protocol version defaults will be used by .NET Framework 4.x. If the value is set to 1, then .NET Framework 4.x will inherit its defaults from the Windows Schannel DisabledByDefault registry values. If the value is undefined, it will behave as if the value is set to 0. By configuring .NET Framework 4.x to inherit its values from Schannel we gain the ability to use the latest versions of TLS supported by the OS, including TLS 1.2.

From Notepad.exe, create a text file named NET4X-UseSchannelDefaults.reg.

Note: When configuring a system for TLS 1.2, you can make the Schannel and .NET registry keys at the same time and reboot the server once.

Validating TLS 1.2 is in use and identifying older incoming connections.

Once TLS 1.2 has been enabled it may be helpful to validate your work was successful and the system is able to negotiate TLS 1.2 for inbound (server) connections and outbound (client) connections. We will provide a few methods for validating this.

HTTP Based Protocols

Many protocols used in Exchange Server are HTTP based, and therefore traverse the IIS processes on the Exchange server. MAPI/HTTP, Outlook Anywhere, Exchange Web Services, Exchange ActiveSync, REST, OWA & EAC, Offline Address Book downloads, and Autodiscover are examples of HTTP based protocols used by Exchange Server.

Windows Server 2016 and Windows Server 2012 R2

The IIS team has added capabilities to Windows Server 2016 and Windows Server 2012 R2 to log custom fields related to encryption protocol versions and ciphers. We recommend reviewing the following blog for documentation on how to enable these custom fields and begin parsing logs for information on incoming connections in your environment related to HTTP based protocols.

Windows Server 2008 through 2012

Unfortunately, the IIS custom fields mentioned above do not exist for Windows Server 2008 SP2 through Windows Server 2012. You may have to rely on alternate methods to validate TLS 1.2 is in use on these versions of Windows Server for HTTP based protocols. Your load balancer or firewall logs may be able to provide this information. Please request guidance from your vendors to determine if their logs may provide this information.

Message Headers (Exchange Server 2016 Only)

Message header data in Exchange Server 2016 provides the protocol negotiated and used when the sending and receiving host exchanged a piece of mail. While this is a more manual method of checking how mail arrived it can be used for testing between specific systems in a pinch.

Note: There is one known exception to the message headers example. When a client sends a message by connecting to a server using authenticated SMTP (also known as the SMTP client submission protocol), the TLS version in the messages headers does not show the correct TLS version used by a customer’s client or device. Microsoft is investigating the possibility of adding this information in a future update.

Mail Flow via SMTP Logging

SMTP Logs in Exchange 2010 through Exchange 2016 will contain the encryption protocol and other encryption related information used during the exchange of email between two systems.

When the server is the SMTP receiving system, the following strings exist in the log depending on the version of TLS used.

TLS protocol SP_PROT_TLS1_0_SERVER

TLS protocol SP_PROT_TLS1_1_SERVER

TLS protocol SP_PROT_TLS1_2_SERVER

When the server is the SMTP sending system, the following strings exist in the log depending on the version of TLS used.

POP/IMAP

No logging exists which will expose the encryption protocol version used for POP & IMAP clients. To capture this information, you may need to capture netmon logs from your server or inspect traffic as it flows through your load balancer or firewall where HTTPS bridging is taking place.

In many deployments by the time client connections reach the Exchange Server, the source IP of the incoming client connection has been replaced with the IP address of your load balancer or firewall. While you are still able to identify if TLS 1.2 is being used by these connections and validate your servers are operating properly, you may be unable to identify exactly what machine is responsible for the incoming client connection if it is still using older TLS protocol versions. In this scenario you may need to request guidance from the vendor of your load balancer of firewall to be able to parse the logs of those devices to find the true IP of the incoming connection, so you may ensure those machines are also properly updated and configured.

Additional Considerations

There are many more considerations beyond Exchange Server when making any changes to cryptography settings within an environment. Considerations such as (but not limited to):

Do your third-party or custom in-house applications that integrate with Exchange Server or Office 356 support TLS 1.2?

The point to take home here is while we can provide guidance for interacting with Office 365, Exchange Server, and other Microsoft products - we cannot guarantee everything in your environment will be unaffected. As such we strongly recommend any steps you take to transition to TLS 1.2 and away from older security protocols are first performed in labs which simulate your production environments before you slowly start rolling them out in production.

Action Items

Configure your Exchange Servers so they can use TLS 1.2 for incoming and outgoing connections using the steps provided and validate the protocol is actively being used.

Start identifying incoming connections using older versions of TLS after TLS 1.2 has been enabled and make plans for those clients if you intend to disable older TLS protocol versions. Remember, a “client” in these terms could be another server device but when we see it as an incoming connection to an Exchange Server we consider the host initiating the connection to be operating in the role of a client.

Deploy the latest releases for Exchange 2010, Exchange 2013, and Exchange 2016 released in March 2018. These releases are the first to support turning off TLS 1.0 and TLS 1.1. Guidance on how to do this will be made available in Part 3 of this blog post series.

Review

With proper execution, you will be able to enable the TLS 1.2 protocol on any Exchange Server running an Exchange Server Cumulative or Update Rollup released after September 1, 2017 installed on OS’es as far back as Windows Server 2008 SP2. With all your Exchange Servers able to use TLS 1.2 for incoming and outgoing connections you should be well prepared for the eventual sunsetting of older TLS protocols. Equally important is understanding what incoming connections in your environment are not utilizing TLS 1.2 now that it is available and building a plan for each of those systems if you intend to disable the older TLS protocols.

Thank you for the guide, Brian. I have a question regarding the validation on SMTP logs. We are on RU5, and currently I do not see any TLS information in the logs other than START TLS but I can see that some of the connectors are using TLS 1.2 via O365 logs. Is this information added as part of the RU22?

Hi Brian, if you get a chance, can you take a peek at ticket 11463992 – we are seeing instances in our tenant where headers report TLS1.2 as being used, but checking a message trace, or EMT we see it was in fact TLS1.0 – this has meant all the checks we have done for systems needs to be completely re-done due to the fact headers appear to not always accurately report the TLS version being used. (we have tested using systems incapable of TLS1.2 and had the same results) – Happy to discuss further if you have the time.